1. Field of the Invention
The present invention relates to a power amplifier system for use in, for example, a car audio set, and more particularly, to a power amplifier system provided with a protection function in case the GND terminal is open and the output terminal is grounding (referred to as “GND open and output grounding”, hereinafter).
2. Description of the Related Art
FIG. 5 shows a block diagram of a power amplifier system that is installed in a car audio set. As shown in FIG. 5, a power amplifier IC 50 includes, as terminals for external connection, a power terminal (Vcc terminal) 11, a ground terminal (GND terminal) 12, an output terminal 13, a ripple terminal 14, etc. Between the power terminal 11 and the ground terminal 12 of the power amplifier IC 50, an external direct-current power supply (battery) 21 is connected, and also a control IC 22 having an external power IC, a microcomputer, etc. for starting up the power amplifier IC 50 is connected (FIG. 5 shows the state in which the battery is incorrectly connected, as will be explained later). Furthermore, a ripple filter capacitor 23, or a capacitor 23 for a ripple filter, for smoothing a bias voltage is connected between the ripple terminal 14 of the power amplifier IC 50 and the ground, and a loudspeaker, not shown, is connected to the output terminal 13. A control signal output from the control IC 22 is supplied to a bias circuit 19.
In the power amplifier IC 50, a push-pull type power amplifier circuit 16 is connected between the power terminal 11 and the ground terminal 12. This push-pull type power amplifier circuit 16 is composed of a PMOS type push side output transistor PT and an NMOS type pull side output transistor NT, which are serially connected. The connection node of these transistors, that is, the push-pull output node of the power amplifier circuit 16 is connected to the output terminal 13. In FIG. 5, “D” is a parasitic diode located between the drain and source of the output transistor NT connected to the ground potential side. Furthermore, these output transistors PT, NT have their gates connected to a driver circuit 16a. 
A negative potential detection circuit 17 detects that the output potential of the power amplifier circuit 16 becomes negative, and the negative potential detection output node thereof is connected to a gate impedance control circuit 52.
The bias circuit 19 is controlled to be started up by a control signal output from the control IC 22. The bias circuit 19 supplies a bias voltage to the driver circuit 16a so as to make the power amplifier circuit 16 perform operation, and the bias voltage output node thereof is connected to the driver circuit 16a as well as to the ripple terminal 14.
The gate impedance control circuit 52 is connected between the gate and source of the push side output transistor PT connected to the power supply side, and is controlled by the negative potential detection circuit 17.
When the user installs thus configured power amplifier system in a car audio set, there may be often raised errors in connecting or wiring the power amplifier IC 50 and the battery 21. For example, as shown in FIG. 5, in case the ground terminal of the battery 21 is incorrectly connected to the output terminal 13 of the power amplifier IC 50, the ground terminal 12 of the power amplifier IC 50 is not connected to the ground terminal of the battery 21 to come into open state (GND open and output grounding state). In this case, also the ground terminal of the control IC 22 is not connected to the ground terminal of the battery 21.
In this incorrectly connected state, superficially, it is considered that the control IC 22 and the power amplifier IC 50 are not started up since the ground terminal 12 thereof are open. However, as shown in FIG. 5, since an operating current flows through the ground terminal 12, parasitic diode D, and output terminal 13 of the power amplifier IC 50, undesirably, the control IC 22 is started up correctly. That is, the control IC 22 sends a control signal to start up the power amplifier IC 50 without detecting the incorrect connection. Accordingly, regardless of the incorrect connection, the bias circuit 19 of the power amplifier IC 50 is undesirably started up.
At the same time, the negative potential detection circuit 17 detects that the potential of the output terminal 13 of the power amplifier IC 50 becomes negative, and the gate impedance control circuit 52 operates due to the detection output. At this time, as described above, since the bias circuit 19 is operating, it is determined whether or not the push side output transistor PT can be protected depending on the operation state of the gate impedance control circuit 52.
For example, in case the sensitivity of the gate impedance control circuit 52 is set higher and the gate impedance is suppressed to the minimum, the push side output transistor PT can be protected without breakdown. However, in case the sensitivity of the gate impedance control circuit 52 is set higher, when the power amplifier IC 50 is correctly connected to the power supply to drive loudspeakers, not shown, there may be raised a case in which the negative potential detection circuit 17 is made to malfunction due to the back electromotive force of the loudspeakers. In this case, the push side output transistor PT is made to cut off, and the sound quality is made deteriorate.
In contradiction to the above, in case the sensitivity of the gate impedance control circuit 52 is set lower and the gate impedance is made larger, the bias circuit 19 is started up under the incorrect connection state. Thus, when the gate impedance control circuit 52 is operated by the negative potential detection circuit 17, the push side output transistor PT cannot be protected sufficiently.
Accordingly, in the circuit shown in FIG. 5, so as to secure the sound quality when the power amplifier IC 50 is correctly connected as well as the protection intensity of the push side output transistor PT at the time of the GND open and when the output terminal is grounding, it is necessary to set up the gate impedance control circuit 52 and the negative potential detection circuit 17 carefully. However, since elements in a chip are subject to fluctuation depending on the manufacturing process, the design margin is extremely restricted with respect to the design of respective circuits.
For this reason, there is desired a power amplifier system that can surely prevent the breakdown of a push side output transistor without deteriorating the sound quality of an output signal of a power amplifier IC even if a microcomputer etc. for controlling the power amplifier system sends a control signal for start-up operation to the power amplifier IC when the power amplifier IC is incorrectly connected to come into the GND open and output grounding state.
In the Jpn. Pat. Appln. Laid-Open Publication No. 2004-112019, there is disclosed “grounding protection apparatus for sound power amplification apparatus”. In this apparatus, a voltage detection unit detects an applied power supply voltage from a power supply unit to be applied to a sound power amplification circuit unit, and a voltage between unit ground terminals. A control unit generates a switch drive signal to turn on a switch circuit when being judged to be appropriate, and turn off the switch circuit when the sound power amplification circuit unit is judged to be grounding, on the basis of the detection result of the voltage detection unit.
In the Jpn. Pat. Appln. Laid-Open Publication No. 2001-7659, there is disclosed “power amplifier”. This power amplifier is formed by a power transistor circuit of the push-pull configuration whose last stage is supplied with a direct current bias. This power amplifier has built therein a grounding breakdown prevention circuit for preventing the flow of an overcurrent by cutting off the direct current bias of a power transistor when the loudspeaker drive output end comes into grounding.